About the image: NASA's Galileo spacecraft acquired this view of Io in July 1999, and approximates what the human eye would see. Credit: NASA/JPL-Caltech/University of Arizona

About the image:NASA's Galileo spacecraft acquired this view of Io during a close fly-by in 1999. Credit: NASA/JPL/University of Arizona

On the Back

Jupiter's rocky moon Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high.

Io’s remarkable activity is the result of a tug-of-war between Jupiter's powerful gravity and smaller but precisely timed pulls from two neighboring moons that orbit farther from Jupiter – Europa and Ganymede.

In mythology, Io is a mortal woman transformed into a cow during a dispute between the Greek god Zeus – Jupiter in Roman mythology – and his wife, Hera – Juno to the Romans.